Liquid hollow cathode lamp



Jan. 14, 1969 c. R. SEBENS ET AL 3,422,301

LIQUID HOLLOW CATHODE LAMP Filed June 24, 1966 INVENTORS. ari Selveus BYJ Vollmer III'IWRNIY.

United States Patent 7 Claims This invention relates to a new type ofhollow cathode used in a lamp, especially useful as a light source inspectroscopic instruments of the atomic absorption type.

In atomic absorption spectroscopy the sample is analyzed by determiningthe absorption (at a certain specific wavelength of radiation) caused bythe atoms for which the analytical test is being made. This technique isparticularly useful for analyzing (both qualitatively andquantitatively) a sample containing one or more metals metalliccompounds. Usually the metallic sample is converted into a salt (ifnecessary), then is dissolved in a liquid solvent (such as water), andis then vaporized in the flame of a burner, so that the sample isatomized. The atomized sample is then irradiated with a light sourcewhich is of great intensity at least at one characteristic absorptionban-d of the metal for which the test is being made. Only radiation inthe region of this characteristic wavelength which is passed through thesample is then allowed to affect a detector, which therefore yields ameasurement of how much absorptoin has occurred. The detected intensity(as compared to the original source intensity, for example) thereforeyields a quantitative measurement of the concentration of the particularmetal for which the analysis is being conducted.

In order to irradiate the sample at high intensity in the narrowabsorption band, the light source itself preferably includes arelatively high concentration of the metal for which the test is beingmade. At the present time the typical such light source is the hollowcathode lamp, in which a cup-shaped element (including at least asubstantial percentage of the metal for which the test is intended) actsas the negative electrode of the lamp. Both this hollow cathode and thepositive electrode are hermetically sealed within a glass envelope in alow pressure atmosphere of an inert (noble) gas.

Ideally a hollow cathode should produce the desired spectral band athigh intensity, without substantial intensity fluctuations with time,and have a long useful life. Thus, the cathode material must be able towith stand the relatively high temperatures necessarily developed whenelectrical currents are passed therethrough (as is necessary to developthe high intensity radiation). In particular the cathode material shouldnot boil, sublime, unduly sputter, decompose or change its relativecomposition (i.e., the various constituents should main tainsubstantially the same proportions before and after a reasonable periodof use). Ideally the cathode material should also be capable of beingformed into the desired cup shape relatively readily.

For those metals intrinsically having the desired properties (such as: amelting point above, say, about 500 C.; moderate vapor pressure in theneighborhood of this temperature; good machina'bility and othermechanical characteristics, and the like), the hollow cathode may becomposed of the pure metal desired (such as copper, silver, and manyother metals having the above-mentioned properties). When the materialdesired to be incorporated in the hollow cathode lacks one or more ofthe desired properties, other techniques must be utilized to obtain asatisfactory cathode including these materials. One possible techniqueis the utilization of the desired metal in the form of a mixture oralloy (both terms being used in their broadest sense) with one or moreother metals or other materials.

For example if the particular desired metal has a relatively low meltingpoint, one technique is to form a close alloy (such as an intermetalliccompound) with one or more of the metals, so as to raise its effectivemelting point above the 500 or 600 C. maximum operating temperature ofthe lamp. Unfortunately such a technique generally reduces the intensityof the emitted radiation caused by the atoms of the first desired metal.

1 The invention avoids this disadvantageous effect by utilizing arelatively thin coating of the low melting point desired metal in theinterior of a hollow cathode. Specifically it has been found thatcertain metals, having melting points below the cathode temperaturesduring normal operation of the lamp, may nevertheless be util-izedintheir pure state as a coating over interior walls of a modified hollowcathode cup. Specifically it has been found that by turning in the endsof the integral walls of the hollow cathode cup (as by swaging), certaintypes of pure metals may be retained within the cup despite the factthat the cathode temperature during operation is above the melting pointof the particular metal.

The invention is particularly adaptable to those metals having arelatively low melting point but a substantially higher boiling point.In particular it is important that the metal, although liquid, has arelatively low vapor pressure at lamp operating conditions, in order tomaintain relatively low loss rates of the metal. The invention isdirectly applicable to otherwise conventional hollow cathode lamps, onesuch lamp being shown for example in applicants co-pending applicationSer. No. 510,754, filed Dec. 1, 1965 (in FIG. 4 thereof). The inventionrequires merely the use of the correct material to form the hollowcathode, the relatively simple operation of turning in the-ends of thecylindrical wall forming the opening of the hollow cathode cup (e.g., byswaging), and the coating of the interior walls of the resultingmodified hollow cathode cup with the desired metal (assuming it has theabove-mentioned properties of low vapor pressure,

even though molten, at the operating temperatures of the lamp).

The relatively simple structural changes required by the invention leadto extremely significant improvements in the efiective brightness of thelamp. For example, hollow cathode lamps in which the metal coated on theinterior of the modified hollow cathode cup was tin, have exhibitedthirty times the brightness (over a relatively long and useful life) ofconventional lamps, which latter typically utilize either pure tin runat relatively low currents to avoid melting or less frequently a tinalloy (also run solely as a solid).

An object of the invention is therefore the provision of an improvedhollow cathode lamp in which the emitting material of the cathodeexhibits greatly enhanced brightness without materially reducing thelife expectancy of the lamp.

Another object is the provision of a hollow cathode lamp having theabove desired properties, in which the emitting material is in themolten state during operation of the lamp.

Other objects and advantages of the invention will become obvious to oneskilled in the art upon reading the following detailed description inconjunction with the accompanying drawing, in which:

FIG. 1 is an enlarged vertical section through the longitudinal axis ofthe improved hollow cathode of the invention; and

FIG. 2 is a vertical section taken on the line 2-2 in FIG. 1.

A completed hollow cathode assembly is shown generally at this cathodemay be used in various known hollow cathode lamps, for example, the oneshown in applicants above-mentioned co-pending application. This hollowcathode assembly comprises the cathode holder or cup 12, including anarrower support portion 14 having at 16 a cylindrical aperture forreceiving the supporting and electrical connecting pin of the lamp. Thelarger body portion of the cathole holder 12 comprises a circumferentialwall portion 18 surrounding the longitudinal (i.e., horizontal in thedrawing) axis of the holder, thereby having a generally cylindricalinterior wall at 20 enclosing the generally tubular interior volume ofthe holder. In distinction to conventional hollow cathode holders orcups, the circumferential wall portion 18 is not cylindrical to its open(i.e., right-hand) end, but rather includes end portion 22 which isin-turned toward the longitudinal axis of the cup so as to formin-turned portion 24 of the interior wall.

Substantially the entire interior wall '20 is coated, as shown at 30,with the pure metal, the emission of which is desired to be obtained inthe hollow cathode lamp. As indicated at 32, this metallic coating issomewhat thicker at the lower part of interior wall portion 20 than thecoating is (at 34) on the upper part of the interior wall (see FIG. 2).This relatively greater thickness of the lower parts of the coatingoccurs since the metallic coating is molten during the use of the hollowcathode in the lamp (as will be more fully described hereafter), so thatmore of the metallic material tends to settle to the lower parts becauseof gravity. Since the hollow cathode holder 12 forms part of theelectrical circuit to the emissive coating 30, the holder is whollycomposed of metal (which in itself is conventional). The particularmetal of which the holder 12 is composed, however, should form no alloyshaving a relatively low melting point with the particular emissivecoating metal employed in the particular hollow cathode assembly, evenat the relatively elevated temperatures (e.g., up to about 500 C.) towhich the assembly may be raised during operation. In fact, if thecoating metal and the material of the cathode holder alloy at all, theresulting alloys should be of low content as to the coating metal. Thisavoids reducing the amount of this coated metal available for emissioneven after relatively long use. Thus only a thin inter-layer of alloy,having a small proportion of the coated metal forms between the coatingand the internal surface 20 of the cathode holder material. The inturnedend of portions 22 of walls 18 assist in maintaining the coatingmaterial 30 within the interior of the cathode when the coating is inmolten state, even if the hollow cathode is tipped slightly, so as tolower its right-hand or open end.

Lamps utilizing hollow cathodes conforming generally to that shown inthe drawing have been made and successfully tested, in which the coating(at 30) were, respectively, tin, gallium, and indium. Since each of thehollow cathodes is somewhat diflerent, each one will be describedseparately.

Example I.-Tin

cathode cup is attached to a conventional stem assembly of a lamp byintroducing the center pin thereof into cylindrical aperture 16 andcrimping the surrounding support portion 14 thereon. Since the variousassembly steps form no part of the present invention, a completedescription is not included here, especially since this is fairlycompletely described in the above-mentioned c0- pending application.Various conventional baking and flushing steps (as described in saidco-pending application) are normally performed. After such conventionalsteps, a small quantity of pure solid tin is added to the interior ofthe cup 12 (the cup being positioned with its longitudinal axisvertical, so that opening 28 is at the top). It has been found that oneor two round pellets or shot of one-eighth inch diameter (one of theforms in which pure tin is commercially available) is entirelysatisfactory. The rest of the conventional assembly, flushing and bakingsteps are then finished (without allowing the tin shot to leave thehollow cathode cup). The completed lamp is then processed in a more orless conventional manner; this consists in operating the lamp forvarious relatively short periods of time while evacuating andreplenishing the gas (preferably neon) between each run. Aftercompletion of this run-in, fresh neon at a few millimeters (of mercury)pressure is fed into the evacuated lamp, and the lamp is finally sealed.

After completion of assembly, out-gassing, and sealing of the entirelamp, the lamp (with the open end 28 of the hollow cathode stilluppermost) is connected to a suitable source of electrical energy andrun at slightly higher than normal operating conditions (in theparticular lamp where 25 ma. is normal operating current, approximately30 ma. is used during this step). Under such elevated temperatureconditions the tin shot will melt so as to form a pool in the bottom ofthe cathode cup. The entire lamp assembly is then tilted so that itsaxis (and therefore the longitudinal axis of the hollow cathode) isvariously between about 45 and (relative to the vertical), and the lampis then rolled about its longitudinal axis so as to cause the tin to wetthe interior surface 20 of the titanium holder all the way up to thebeginning of the turned-in (swaged) end portion 22; this point isindicated at 26 in FIG. 1. The electrical energy source is then turnedoff, and the rotation of the lamp continued while the tin solidifies.

Tin lamps made according to the above description are normally operatedbetween 20 and 30 ma. at about 200 volts -D.C. Lamps constructed and runin the disclosed manner exhibit a brightness of approximately thirtytimes that for conventional tin hollow cathode lamps (in the region of2246 A., the atom line wavelength usually used in atomic absorptionspectroscopy for tin). In such conventional lamps the tin is present inthe form of either pure tin machined or cast in a holder, the metal ofwhich would alloy with the tin if the latter were molten (for example, acopper holder), or an alloy of tin which is solid throughout operationof the lamp. The fact that the pure tin is liquid during the operationin the new lamp utilizing the hollow cathode of the invention thusyields substantial performance improvement. Atomic absorptionspectroscopy tests of the inventive liquid tin hollow cathode lamp usedwith a Model 303 Perkin-Elmer atomic absorption spectrophotometeryielded a maximum sensitivity (for 1% absorption) of 1.2 p.p.m. of tinin the sample, with as little as 0.2 p.p.m. tin being detectable. Forthis maximum sensitivity, the liquid tin lamp was run at 40 ma.; and theinstrument was set at 10 scale expansion, a noise suppression setting of5, and a spectral slit width of 2 A., (slit position UV3). The useablesensitivity and detection limit are of course slightly lower when thelamp is run in the range of 20-25 ma., which affords useful life in themany hundreds of hours (for example, 500-600 hours); however, reasonableuse of the lamp at its maximum practical brightness of 40 ma. reducesits useful life only by a moderate factor (approximately to one half, or250-300 hours).

Because of the fact that the tin completely melts during operation (theinterior of the hollow cathode reaches temperatures of approximately 300C. at 20 ma), the liquid tin film (at 30) will tend to be thicker at thebottom (i.e., at 32) than at the top 34 when the lamp is used in itsnormal position with its longitudinal axis horizontal. For this reasonthe coating will appear as shown in the drawing even when the lamp iscool between uses. It should be noted that the lamp should not be turnedso that the open end of the hollow cathode (at 28) extends downward,either during use or immediately thereafter (before the cathode hascooled to below the melting temperature of the tin).

Example II.Gallium Lamps having a hollow cathode in which gallium formedthe interior coating have been successfully made, using a columbium(also known as niobium) hollow cathode cup, with which the gallium doesnot readily alloy. After thorough cleaning, the columbium hollow cathodeholder is swaged heavily as indicated at 22 in the drawing. Indistinction to the Example I technique for tin, the gallium isintroduced and cast into the columbium hollow cathode holder prior toassembly of the other parts of the hollow cathode lamp. Since galliumhas a melting point not much above room temperature, the gallium may bemost easily introduced into the hollow cathode holder by warming thegallium to a temperature above 30 C., and by use of an eye dropperplacing one drop (of approximately one-eighth inch diameter) in thecolumbium holder while the latter is positioned with its opening 28uppermost. The holder and drop of gallium are then placed in this samevertical position on a centrifuge. The holder is covered (as with aPyrex tube) and argon gas is used to continuously flush the assemblyduring the following steps. With the centrifuge operating, the cathodeis heated (with an induction heater) to approximately 800, the galliumthereby wetting the cylindrical walls 18 of the columbium holder andactually climbing up these walls toward the swaged, turned-in endportions 2'2. When the gallium has climbed up the walls to about thepoint 26, the heat is removed (with the centrifuge still spinning). Whenthe cathode assembly has cooled to about 100 C., it is removed from thecentrifuge and any excess gallium is allowed to flow out by invertingthe cathode holder.

After the cathode assembly has cooled to room temperature, it may becrimped onto the center pin of the stem assembly of the lamp in aconventional manner and the rest of the lamp assembled in theconventional manner generally described above in Example I (argon beingused as the flushing gas). The lamp is then run in by repetitiveoperation under more or less normal operating conditions, but withcomplete evacuation and fresh filling with neon gas after each run. Thefinal filling is with neon at a few millimeters (of mercury) pressure.Neon is preferred to argon as the filling gas because the latter has anabsorption line very near one of the two most useful gallium lines,namely 2944 A.

Lamps made according to the above description are normally operated at20 ma., and are preferably used with an atomic absorption spectrometerat either a 2874 or a 2944 A., wavelength setting (there also beingsubstantially lower sensitivity absorption lines at, for example, 4033and 4172 A.) The nominal sensitivity (in ppm. producing a 1% absorptionreading) was 2.3 p.p.m. at 2874 A. and 2.4 p.p.m. at 2944 A., with theabove-mentioned Model 303 Perkin- Elmer atomicabsorptionspectrophotometer, using a narrow UV3 (2 A.) slit setting. Atrelatively high concentrations of gallium and relatively large slitwidths, the sensitivity at 2874 A. decreases less rapidly than that at2944 A.; additionally the 2874 A. line has somewhat less backgroundnoise. Therefore under many conditions the 2874 A. line is preferablyused. The lower detection limit was 0.07 ppm. at 2874 A. (using theabove spectrometer at a wide -UV5 (20 A.) slit setting, high noisesuppression setting of 4, scale expansion of 30X, and a speciallymodified burner with a test sample containing one ppm. gallium). Liquidgallium hollow cathode lamps according to the above description havebeen successfully made and have shown relatively long, useful lives(i.e., over 1,000 hours).

Example III.-Indium The technique for making a hollow cathode lampaccording to the invention in which indium is the active coating withinthe cathode holder is very similar to the gallium lamp of Example II.The major difference is that a titanium holder is used (as in Example I,Tin). Specifically, the titanium holder is swaged heavily (as in theprevious examples) after celaning; a solid piece of indium in the formof the Ms" diameter shot (cut to this size if necessary) is then placedin the cup, and the assembly placed with the open end (at 28) uppermostin a centrifuge and covered with a quartz tube. Argon is flowed over thecathode assembly within the tube (at about 5 cu. ft. per hour). With thecentrifuge operating, the titanium is heated to a dull orange heat(about 900 C.) with an induction heater. At this temperature the indiumreadily wets the titanium so as to climb up the now vertical interiorsurface 20 of the cylindrical wall 18. The centrifuging and heating iscontinued until the indium has climbed to at least point 26. at whichtime the heat is removed. The spinning of the centrifuge is continueduntil the indium has hardened, the argon gas flow being maintained untilthe temperature has closely approached room temperature.

The rest of the assembly steps of the lamp structure are conventional,being the same as those for the gallium lamp of Example II. It may benoted that in all of these techniques substantially all of the steps aredone with argon as a forming gas surrounding the hollow cathode. After aleak check, the lamp is evacuated and baked for a few hours at a fewhundred degrees (C.) and then gradually cooled (this baking step beingconventional and common to the manufacture of all of the exemplary lampsdisclosed). The lamps are then run in with neon gas, in a manner similarto the gallium lamps (with complete replacement of the neon gas aftereach run). After the last run the lamp is filled with fresh neon to apressure of a few millimeters, and then finally sealed. To insure evencoating of the indium on the interior surface of the titanium holder,the lamps may be run at 30 ma. to completely melt the indium; and thelamp then rolled at betwen 45 and angle (in the same manner as explainedin Example I for tin). As before, the rotation of the lamp should becontinued after the turning off of the current until the lamp has cooledbelow the melting point of indium.

Indium lamps according to the above example, when run at a current of 20ma., have successfully undergone life tests of over 500 hours. At thissame normal operating current, use with the previously mentionedspectrophotometer yielded a sensitivity of 0.9 ppm. indium (to produce1% absorption) both at the 3256 A. (using a slit wid.h setting of UV3,which is equal to 2 A.) and at the 3039 A. (a UV4 slit setting of 7 A.)absorption lines. The ultimate detection limit when using an indium lampaccording to the invention is approximately 0.05 ppm. indium (using ascale expansion factor of 30X at a noise suppression setting of 4 with atest solution containing 0.2 ppm. indium). It is emphasized that thisvery good sensitivity is obtained at the normal operating current valueof 20 ma.

The three above examples and the results obtained therewith clearly shownot only the feasibility, but the actual improvement occasioned by theinvention. Specifically, using a hollow cathode holder of a materialwhich does not substantially alloy with the desired emitting metal usedas its coating (at 30), which holder has been modified by the inclusionof in-turned portions 22 allows the various coated metals on theinterior surface 20 to be retained even at temperatures substantiallyabove their melting points. As noted above, the invention may be usedwith such coatings formed of any metal which has a relatively low vaporpressure under the operating conditions of the lamp, even though it isliquid. As specifically pointed out previously, the hollow cathodeholder material should be chosen so that the coated metal forms with thematerial of the cathode holder neither a low melting point alloy nor anyalloy having a relatively large amount of the coating metal, even at themoderately high temperatures reached during operation of the lamp (e.g,,300 400 C.). A tendency to form a thin intermediate layer of arelatively high melting alloy (at least at the somewhat highermanufacturing temperature), which alloy contains only a few percent (atmost) of the coated metal, is actually advantageous. Specifically such athin intermediate layer assists both in forming the original coating 30over the entire interior surface 20 of the cathode holder and inmaintaining during use (when the coating is liquid) at least a thincoating even where (as at 34) gravity tends to cause the coating to run.

Markedly greater brightness is obtained by the lamps of the inventionutilizing the tested-for metal in its pure molten state. Contrary toexpectation, the molten coating does not form a single globule of moltenmaterial, even though the material is completely liquid under operatingconditions, but rather maintains a relatively complete coating of theinterior surface 20 of the cathode holder, with only a moderate tendencyto thicken at the bottom (as indicated at 32). Although three fullytested, specific examples of the invention have been given, (namely,with tin, gallium and indium as the coating metals), the invention isnot limited to these specific materials but is usable for making hollowcathode lamps with metallic coatings (at 30) of other materials havingthe desired properties of a boiling point substantially above theoperating temperature of the cathode and a low or at least moderatevapor pressure at this operating temperature. As above noted, thematerial of the cathode holder should be chosen so that the interiorlycoated metal does not readily alloy therewith to any great extent evenat the somewhat elevated operating temperatures. On the other hand, thematerial of the holder should be at least somewhat wettable by the metalcoating so as to insure a relatively complete coating action andmaintenance of this coating over at least a substantial area of theinterior surface 20 of the cathode cup.

Because of its applicability to other metals, the invention is notlimited to the specific examples hereinbefore described. Since variouschanges in the exemplary manufacturing techniques may be employed, theinvention is not limited to any of the disclosed details, but rather isdefined in the appended claims.

We claim:

1. In a hollow cathode lamp of the type in which the hollow cathodecomprises at least one particular metal for which spectral lines aredesired, such lamps being especially adapted for use in atomicabsorption spectroscopy, the improvement comprising:

said hollow cathode being of generally cup shape, with the wallsdefining its interior having generally inturned wall portions adjacentto the open end thereof;

a thin coating of said particular desired metal substantially coveringthe entire interior surface of said walls at least up to said in-turnedend portions,

said particular desired metal having a melting point below the normaloperating temperature of said lamp, but having a boiling pointsubstantially above said temperature and a relatively low vapor pressurethereat; said in-turned end portions of said wall thereby inhibitingloss of the liquid metal coating during operation of said lamp; wherebyrelatively high intensnity radiation at the spectral emission-absorptionlines of said particular desired metal is obtained. 2. An improvedhollow cathode lamp according to claim 1, in which:

said particular desired metal coating comprises substantially pure tin.3. An improved hollow cathode damp according to claim 2, in which:

said generally cup-shaped hollow cathode comprises titanium; wherebyalloying between said tin coating and said titanium hollow cathode isminimized, even at the moderately elevated operating temperature of saidlamp. 4. An improved hollow cathode lamp according to claim 1, in which:said particular desired metal coating comprises substantially puregallium. 5. An improved hollow cathode lamp according to claim 4, inwhich:

said generally cup-shaped hollow cathode comprises columbium (niobium);whereby alloying between said gallium coating and said columbium hollowcathode is minimized, even at the moderately elevated operatingtemperature of said lamp. 6. An improved hollow cathode lamp accordingto claim 1, in which:

said particular desired metal coating comprises substantially pureindium. 7. An improved hollow cathode lamp according to claim 6, inwhich:

said generally cup-shaped hollow cathode comprises titanium; wherebyalloying between said indium coating and said titanium hollow cathode isminimized, even at the moderately elevated operating temperature of saidlamp.

References Cited UNITED STATES PATENTS 2,161,790 6/1939 Abadie 3 l3-3462,810,089 10/1957 MacNair 313346 X 2,847,605 8/1958 Byer 3133463,089,054 5/1963 Walsh et al. 313218 X 3,286,119 11/1966 Sugawara et a1.313-217 X FOREIGN PATENTS 977,545 12/ 1964 Great Britain.

JOHN W. HUCKERT, Primary Examiner.

J. R. SHEWMAKER, Assistant Examiner.

US. Cl. X.R. 313346

1. IN A HOLLOW CATHODE LAMP OF THE TYPE IN WHICH THE HOLLOW CATHODECOMPRISES AT LEAST ONE PARTICULAR METAL FOR WHICH SPECTRAL LINES AREDESIRED, SUCH LAMPS BEING ESPECIALLY ADAPTED FOR USE IN ATOMICADSORPTION SPECTROSCOPY, THE IMPROVEMENT COMPRISING: SAID HOLLOW CATHODEBEING OF GENERALLY CUP SHAPE, WITH THE WALLS DEFINING ITS INTERIORHAVING GENERALLY INTURNED WALL PORTIONS ADJACENT TO THE OPEN ENDTHEREOF; A THIN COATING OF SAID PARTICULAR DESIRED METAL SUBSTANTIALLYCOVERING THE ENTIRE INTERIOR SURFACE OF SAID WALLS AT LEAST UP TOSAID-IN-TURNED END PORTIONS, SAID PARTICULAR DESIRED METAL HAVING AMELTING POINT BELOW THE NORMAL OPERATING TEMPERATURE OF SAID LAMP, BUTHAVING A BOILING POINT SUBSTANTIALLY ABOVE SAID TEMPERATURE AND ARELATIVELY LOW VAPOR PRESSURE THEREAT; SAID IN-TURNED END PORTIONS OFSAID WALL THEREBY INHIBITINH LOSS OF LIQUID METAL COATING DURINGOPERATION OF SAID LAMP; WHEREBY RELATIVELY HIGH INTENSNITY RADIATION ATTHE SPECTRAL EMISSION-ABSORPTION LINES OF SAID PARTICULAR DESIRED METALIS OBTAINED.